Why in Situ in rat works but not in mouse

In general I am doing in situ on mouse tissue. Now I have a ribo probe that is not working in mouse but perfectly in rat. The rats and mice are fixed and embedded the same way and also the in situ was done the same way for both species.

Does anybody base the slightest idea why this can bee and how I might get the in situ start working in mouse.

I am not sure if you are still waiting for an answer to your question. I am doing ISH and I am also experiencing some problems. Did you check that the probe you are using is specific to mouse? If it is a probe designed for a rat gene, that might be the reason it doesn't work in mouse?